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Source parameters and scaling relationships of small earthquakes in the northeastern Caribbean

LAMONT-DOHERTY GEOLOGICAL OBSERVATORY AND DEPARTMENT OF GEOLOGICAL SCIENCES COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964

Abstract

The seismic moments and stress drops of 23 earthquakes (1.1 M 2.4) that occurred during an earthquake swarm in the Virgin Islands were determined from the analysis of their P waveforms. The data consist of digitally recorded seismograms collected by a short-period seismic network operating in the northeastern Caribbean. The events of the swarm are particularly useful for comparing the relative stress drops of small earthquakes, because their source to receiver paths and focal mechanisms are very similar.

The static stress drops calculated for these earthquakes varied from about 0.2 to 2 bars. The data clearly illustrate that the static and dynamic stress drops of these earthquakes generally increased with the size (moment) of the events. The fault radii for these shocks increased with seismic moment, but only by a factor of 2 for a 100-fold increase in seismic moment. The velocity waveforms of the larger events were systematically more impulsive than those of the smaller earthquakes. These observations imply that, for this set of earthquakes, the final fault radius is a function of the stress drop that occurs during the rupture process.

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